Conventionally, various types of systems for monitoring the condition of a wheel have been known. One of the systems is, for example, a system for monitoring the internal pressure of a tire, which includes a transmitter consisting of a pressure sensor for detecting the internal pressure of tire, which is one of the conditions of wheel, and a transmission circuit for transmitting the pressure data, and a receiver consisting of a receiving circuit for receiving the pressure data sent from the transmitter, and monitors the internal pressure of tire and gives an alarm etc. to the driver if abnormality is found. These wheel condition monitoring systems are configured so that, taking the internal pressure of tire as an example, the pressure data from the pressure sensors installed on the individual wheels are sent from the individual transmitters to one receiver installed on the vehicle body side. The intensity of electric waves that are sent from the transmitter and reaches an antenna of the receiver changes in accordance with a change in position of the transmitter, which is caused by the rotation of wheel. When the transmitter is present at a certain rotation angle, the receiving intensity of electric waves at the receiver becomes low, so that in some cases, a rotation angle at which the transmission and reception cannot be achieved is present.
FIG. 1 is a diagram for explaining one example of the send and receive conditions in the above-described conventional wheel condition monitoring system. In the example shown in FIG. 1, the relative values of receive intensity are plotted for the rotation angle of wheel (360° for one turn of tire), and data can be transmitted and received stably in a region on the outside of the receive limit. In FIG. 1, it is found that a dead point, at which the receive intensity does not reach the receive limit is present at a right lower portion. FIG. 1 shows a concept of one example, and the position of dead point or the presence of dead point changes according to the tire size, data size, data transmission speed, and the like. The above-described example has a problem in that the demodulation of data is impossible at this dead point. In such a situation, the probability of achieving the transmission and reception decreases, and hence the system does not function stably.
In order to eliminate the hindrance to the transmission and reception of data, which is caused by the above-described dead point, and to increase the probability of transmission and reception, generally, the number of times of transmission has only to be increased. However, in the wheel condition monitoring system relating to the present invention, if the number of times of transmission is increased excessively, there arises a problem in that the following disadvantages are provided.    (1) An increase in the number of times of transmission accelerates the run-down of battery, which shortens the life of transmitter.    (2) An increase in the number of times of transmission causes the transmission to overlap with the transmission of electric waves from another tire in terms of time, which sometimes makes the reception of data impossible.
It is an object of the present invention to provide a wheel condition monitoring system capable of performing a stable function of the system by increasing the probability of the transmission and reception even in the presence of a dead point.